In the early 1970s, the American Society for Testing and Materials™ (ASTM) promulgated ASTM D3241 Standard Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels Procedure. This published ASTM standard, most recently revised in 2004, is incorporated herein by reference. The test described in ASTM D3241 measures the thermal stability of jet fuel by subjecting it to test conditions simulating those occurring in jet engine fuel systems.
The ASTM D3241 standard describes the use of an apparatus that, over a period of 2.5 hours, pumps 450 mL of pressurized jet fuel at a fixed volumetric flow rate around and over a single-use long, thin, cylindrical, heated aluminum tube. Such single-use tubes are typically referred to as heater tubes. The color and other characteristics of any fuel oxidation products (i.e., deposits) left on the surface of the central portion of the heater tube, are then evaluated to assess the thermal stability of the jet fuel being tested.
The quality of the surface finish of the single-use heater tube used in the ASTM D3241 test is key to obtaining satisfactory test results. U.S. Pat. No. 3,670,561 (“the '561 patent”), issued Jun. 20, 1972, to Alf Hundere, and which is incorporated herein by reference, illustrates a typical heater tube.
The heater tube used to test jet fuels has a generally circular cross-sectional shape, a substantially constant internal diameter, and an outside diameter that is larger at its extremities than at its central portion. The '561 patent suggests that the heater tube be constructed of aluminum, to minimize manufacturing cost. The outer diameter of the central portion of the tube may be as small as ⅛th of an inch. Furthermore, the '561 patent indicates that the heater tube should have a highly polished surface finish that is consistent from tube to tube and is suitable for rating the level of fuel deposited thereon during a test. U.S. Pat. No. 5,101,658 (“the '658 patent”) and U.S. Pat. No. 5,337,599 (“the '599 patent”) similarly depict and describe a heater tube and are also incorporated herein by reference.
A paper entitled “Comparison of JFTOT Heater Tube Deposit Rating Methods for the Evaluation of Fuel Thermal Stability,” by Robert Morris, et al. of the Naval Research Laboratory, published Dec. 29, 1987, discloses that a standard heater tube can be manufactured from a common aluminum alloy known as 6061-T6 aluminum. 6061-T6 aluminum is an extremely popular alloy because of its low cost, good formability, corrosion resistance, high strength, and attractive appearance.
Disposable heater tubes for testing the thermal stability of fuels have been in existence for more than 30 years. A typical finished heater tube is approximately 6⅜ inches in length, including an approximately 2⅜ inch central portion. The central portion of the heater tube has an outer diameter of approximately ⅛ inch. The remainder of the heater tube has an outer diameter of approximately 3/16 inch.
While the ASTM D3241 standard and the '561, '658, and '599 patents disclose various aspects of the heater tube and the need for a consistent, “highly polished” surface finish, these references do not describe a means, a method, or a best process for obtaining the consistent “highly polished” surface finish. A SAE Technical Paper presented at the Aerospace Technology Conference & Exposition at Long Beach, Calif., between Oct. 14 and 17, 1985, by G. Datschefski and T. G. R. Farthing, entitled “Evaluation of JFTOT Tube Deposits by Carbon Burnoff” indicates that the industry's standard method of creating the surface finish on heather tubes is by the use of a petroleum distillate-based metal polishing liquid containing ammonia.
A metal polishing liquid typically includes an abrasive material suspended in wet lapping. The abrasive material in the metal polishing liquid cuts tiny grooves into the surface of the workpiece being polished. Because these tiny grooves are so small, when a metal polishing liquid is used on a heater tube, the surface of the heater tube appears to the human eye as being bright, reflective, and shiny. Interestingly, the more numerous these tiny grooves, the brighter, shinier and more reflective the surface finish of the heater tube is to the human eye. However, brightness, shininess and reflectivity does not equate to a good surface finish for a heater tube that is used in a jet fuel stability test.
The ASTM D3241 standard test method calls for a visual inspection of the heater tube prior to use. Such visual inspection typically includes checking the surface finish for quality defects and other inconsistencies under a magnifier.
It has long been desired to produce a heater tube with a surface finish superior to that which could be obtained by conventional polishing techniques. Such a heater tube having a superior surface finish would permit advances in studies to understand the mechanism of jet fuel deposit formation on the surface of the central portion of aluminum heater tubes. Use of a heater tube with a superior surface finish would result in a reduction in light scattering from the surface of the heater tube. The reduction in light scattering would permit improved visual observation and study of fuel deposits and their resultant fuel-related effects by reducing the glare associated with heater tubes whose surface finish has been created by the use of abrasive liquid polishing techniques. A heater tube with a superior surface finish would also permit advances in prior art deposit measurement techniques and the development or successful application of new deposit measurement protocols and techniques such as ellipsometry.
One of the techniques that has been evaluated for obtaining a superior surface finish on the central portion of a heater tube is roller burnishing. While roller burnishing has long been practiced as a way of achieving an improved visually pleasing finish on the surface of different items, to include cylindrical workpieces made from aluminum, prior art roller burnishing techniques failed when applied to heater tubes. Specifically, prior art roller burnishing techniques did not produce consistent, high quality surface finishes. For example, in the instruction manual for the CX-1T External Roller Burnishing Machine (copyright 1992), Cogsdill Tool Products, Inc. of Lugoff, S.C., it is indicated that the smaller the workpiece, the greater the speed needed for the roller burnishing tool to obtain a satisfactory surface finish. When this prior art technique of increasing the speed of the roller burnishing tool was tried on aluminum heater tubes, unsatisfactory surface finish results were obtained. Not only is the prior art incorrect with regard to the speed of the roller burnishing tool, but the prior art on roller burnishing also does not provide any instruction concerning the force to be placed on the surface of the heater tube by the burnishing rollers. Nor does the prior art provide any instruction as to understanding when the surface of the workpiece is to be considered as being roller burnished to an acceptable surface finish.
Therefore, there remains a need in the art for a roller burnishing method that will burnish the surface finish of a heater tube.